How to run a perl file in a terminal pipeline - perl

Question

Probably a very simple question:
I have a perl file sed.perl that takes as an input a string, makes some substitutions there and prints it on the standard output.
#!/usr/bin/perl
use warnings;
use strict;
use diagnostics;
use feature 'say';
#use Cwd;
my ($text) = #ARGV;
$text =~ s/\.\)\n/'\.'\)\n/;
print $text;
I want to feed the script with a string output from a terminal pipeline. Let's say in this way:
cat input.txt | perl sed.perl
but this doesn't work: Use of uninitialized value $text in substitution (s///) at
Using a score symbol doesn't works either:
cat input.txt | perl sed.perl -

Answer 1

#ARGV doesn't do what you think it does. It's literally the arguments passed to perl.
E.g. :
myscript.pl some arg
#ARGV will host 'some', 'arg'.
What you want is the STDIN file handle.
e.g.
#!/usr/bin/perl
use strict;
use warnings;
while ( <STDIN> ) {
s/something/somethingelse/g;
print;
}
Now what this is doing is reading STDIN line by line. Your pattern includes \n. Do you actually need it? It looks like you're 'just' using it as a line anchor, and so you could use:
s/\.\)$/'\.'\)/g;
$ is the regex for "end of line" - see perlre for more.
However, as noted in the comments by reinierpost - there's another thing that's probably useful to know - perl has the "diamond operator" <> which does two things:
If filenames are specified to the script, opens them and reads from them.
If no arguments specified, reads STDIN.
So you could do:
while ( <> ) {
s/something/somethingelse/g;
print;
}
And then your script can either be invoked by:
cat input.txt | ./yourscript.pl
Or:
./yourscript.pl input.txt
And you'll have the same result.

Answer 2

You're trying to read the text as if it were an argument, when in fact with a pipeline the information from input.txt will be something you read from stdin. With a pipeline stdout from the left process is connected to stdin of the right process.

Answer 3

As others have mentioned, #ARGV contains arguments to your script, and you'll want to use STDIN instead.
The shortest solution seems to be:
#/usr/bin/env perl
use strict;
while(<>) {
print s/\.\)$/'.')/r;
}
Note that you could also achieve the results of your example using a one-liner as follows:
cat input.txt | perl -pe 's/your/substitution/'