#!/usr/bin/perl
use strict;
use warnings;

my $wanted = 'coucou';

while (my $line = <DATA>) {
    chomp($line);
    my $rest = '';
    if ($line =~ m/$wanted(.*)$/) {
        $rest = $1;
        print "$wanted:$rest\n";
    }
    else {
        print "$wanted not found in $line\n";
    }
}

__DATA__
abc coucou def
not in this line
in this line, but nothing following coucou
coucou once, coucou twice, coucou three times - what should be done with this line?