Assuming your file is large and has to be read line by line, and assuming there may be more than one line with a match:

#!/usr/bin/perl -w

$s1 = 'exp1';
$s2 = 'exp2';

open(FICH, "a.txt");
while (<FICH>) {
    if ($_ =~ /$s1(.*?)$s2/) { print "$1\n"; }
}
close FICH;
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