use strict;
use warnings;
use v5.18;

open (my $fh, "<", 'input.txt');
open (my $fh_out, ">", 'output.txt');

my $prev_line;
foreach $current_line (<$fh>) {

    my $out_line = $current_line;

    if ($current_line =~ /^msgstr/ and $prev_line =~ /^msgid/){

        my ($prev_var) = $prev_line =~ /^msgid\ \"(.+)+\"/;
        my ($cur_var) = $current_line =~ /^msgid\ \"(.+)+\"/;

        if ($cur_var eq $prev_var){
            $out_line =~ s/$cur_var//;
        }
    }

    print $fh_out $out_line;

    $prev_line = $current_line;
}

close($fh);
close($fh_out);