• Check for the success or failure of open and close operations.

• Use the three argument form of open and lexical filehandles.

• Don't keep opening and closeing your output file every time you find a line you are interested in, do them once outside the while loop. Your code keeps opening the file for writing thereby clobbering what you have written in previous loops.

• Add captures to your regex to preserve what comes before and after $element and avoid the performance implications of $'.

• Reading into $_ can save some typing as certain operations default to using $_ if no argument is given.

use strict;
use warnings;

my $element = q{coucou};
my $inFile  = q{EXEMPLE.txt};
my $outFile = qq{RESEARCHED_text_${element}.txt};

open my $inFH, q{<}, $inFile
   or die qq{open: $inFile: $!\n};
open my $outFH, q{>}, $ourFile
   or die qq{open: $outFile: $!\n};

while ( <$inFH> )
{
    next unless m{(.*?)$element(.*);
    print $outFH;
    my $beforeElement = $1;
    my $afterElement  = $2;
    # Do something here with your captured text ...
}

close $inFH
   or die qq{close: $inFile: $!\n};
close $outFH
   or die qq{close: $outFile: $!\n};
[download]

I hope this is of use.

Cheers,

JohnGG