Replace the last line written to a file descritor - perl

Question

I am writing a small perl program where I am checking the pattern of #start and #end. The agenda is to create a separate file with the lines in between start and end patterns. This I am able to do with below script.
#!/usr/bin/perl
open(INFILE,"<","testcases") || die "Can't open file: $!";
my $binary;
my $tccounter=1;
while(<INFILE>)
{
if(/^#start/i)
{
open(OUTFILE,">",$tccounter."_case.sh") || die "Can't open file: $!";
print "start of the script\n";
next;
}
elsif(/^#end/i)
{
################################
# Want to replace the previously
# written line here with some
# addtional customized lines
################################
close(OUTFILE);
$tccounter++;
print "End of the script\n";
print "last line for this testcase is \n $binary\n";
next;
}
else
{
$binary=$_ unless(/^\s*$/);
print OUTFILE $_;
}
}
But what I additionally needed is is identify the last line that is being written to a file and then replace that additional line with some custom data.
For example, here in my case the last line for all the files is execute.
I want replace the line "execute" in all the output files.
In the current output files last line is as below:
execute
expected out files last line should be
preline
execute
postline
Input file (testcases):
#start
line1
line 2
execute
#end
#start
line3
line 4
execute
#end
#start
line5
line 6
execute
#end
#start
line7
line 8
execute
#end

Answer 1

I suggest that you should buffer your output
If you push each line to an array instead of printing it then, once the #end tag is seen, it is simple to locate the last non-blank line in the array and replace it
Then the output file can be opened and the contents of the array printed to it
Here's an untested example
use strict;
use warnings 'all';
open my $fh, "<", "testcases" or die "Can't open input file: $!";
my $n;
my $i;
my $print;
my #buff;
while ( <$fh> ) {
if ( /^#start/i ) {
#buff = ();
$i = undef;
$print = 1;
print "start of the script\n";
}
elsif ( /^#end/i ) {
my $file = ++$n . "_case.sh";
$print = 0;
unless ( defined $i ) {
warn "No data found in block $n";
next;
}
splice #buff, $i, 1, "preline\n", $buff[$i], "postline\n";
open my $fh, ">", $file or die qq{Can't open "$file" for output: $!};
print $fh #buff;
close $fh;
print "End of the script\n";
}
elsif ( $print ) {
push #buff, $_;
$i = $#buff if /\S/;
}
}

Answer 2

I think Borodins answer is the way to go (I'm just not able to comment yet).
So the general algorithm is:
collect full record, from start marker to end marker
once end marker is reached, process record content. In your case:
find last non-empty line and surround it with others
print found line
write out file for record
repeat as needed
I couldn't resist and rewrote Borodins solution using the flipflop operator:
use strict;
use warnings;
open(my $in,'<','in.file') || die "Can't open file: $!";
my ($cnt,#rec);
while( <$in> ) {
push(#rec,$_) if /^#start/i .. /^#end/i; # collect record lines (using flipflop operator)
if( /^#end/i ) { # end of record reached?
next if #rec <= 2; # ignore empty records
# determine index of last nonempty line
my ($lci) = grep {$rec[$_]=~/\S/} reverse (1..$#rec-1); # ...except markers
printf "last line for this testcase is \n%s\n", # print find
splice #rec, $lci, 1, ("preline\n",$rec[$lci],"postline\n"); # surround with pre&post
# write out result
open(my $out,'>',++$cnt.'_case.sh') || die "Can't open file: $!";
$out->print(#rec[1..$#rec-1]); # ...except markers
$out->close;
#rec=(); # empty record for next use
}
}