How to check for a string using Perl given - perl

Question

I'm trying to replace a particular line in a file. I can get my program to run, but it doesn't actually do the replacing that I want it to.
Here is my sample file:
test line 1
test line 2
line to be overwritten
test line 3
Here is the code that I have:
my $origFile = $file_path . "junk\.file";
my $newFile = $file_path . "junk\.file\.backup";
# system command to make a backup of the file
system "mv $origFile $newFile";
#opens the files
open( my $INFILE, $newFile ) || die "Unable to read $newFile\n";
open( my $OUTFILE, '>' . $origFile ) || die "Unable to create $origFile\n";
# While loop to read in the file line by line
while ( <$INFILE> ) {
given ($_) {
when ("line to be overwritten") {
print $OUTFILE "line has been overwritten\n";
}
default {
print $OUTFILE $_;
}
}
}
close($INFILE);
close($OUTFILE);
I've tried to change the when statements several different ways to no avail:
when ($_ eq "line to be overwritten")
when ($_ == "line to be overwritten")
when ($_ cmp "line to be overwritten")
But those only generate errors. Anyone know what I'm doing wrong here?

Answer 1

As highlighted in a comment on the original question, given/when is an experimental feature of perl. I would personally recommend using if/else in a loop, and then either use string equality or a regex to match the line(s) you want to replace. A quick example:
use strict;
use warnings;
while(my $line = <DATA>) {
if ( $line =~ /line to be overwritten/ ) {
print "Overwritten\n";
} else {
print $line;
}
}
__DATA__
test line 1
test line 2
line to be overwritten
test line 3
This will give the output:
test line 1
test line 2
Overwritten
test line 3
You could also use the string equality if you aren't confident in your regex, or the string is guaranteed to be the same:
...
if ($line eq 'line to be overwritten') {
...
Sidenotes
open
On your initial open, it is recommended to use the 3 argument version of open to save from unexpected issues:
open(my $INFILE, '<', $newFile) || die "Unable to read $newFile\n";
(for more info on this, see here: http://modernperlbooks.com/mt/2010/04/three-arg-open-migrating-to-modern-perl.html)
strict & warnings
Also, it is recommended to use strict and warnings in your code file, as seen in my example above - this will save you from accidental mistakes like trying to use a variable which has not been declared, and syntax errors which may give you head-scratching results!
Experimental Features
Experimental features in perl are where there is no guarantee made for backwards compatibility to be maintained when a new release of perl comes out. Obviously if you are using the same version of perl everywhere it should be compatible, but things may break if you update to another major version of perl. answered here as I dont have the reputation to answer in the comments...

Answer 2

You seem to be making it way more complicated than it needs to be - a simple regex to check each line and act accordingly should do the job.
while(<$INFILE>)
{
chomp($_);
if /^line to be overwritten$/ )
{
print $OUTFILE "line has been overwritten\n";
}
else
{
print $OUTFILE "$_\n";
}
}

Answer 3

One way to do it is to use Tie::File module. It allows to replace data right in the file. You can make the backup same way you are currently doing, before changing the original file.
use strict;
use warnings;
use Tie::File;
my $file = 'test.txt';
tie my #textFile, 'Tie::File', $file, recsep => "\n" or die $!;
s/line to be overwritten/line has been overwritten/ for #textFile;
untie #textFile;