use strict;
use warnings;
use Test::More tests=>1;
my $string = 'feeFii foo';
my $regex = qr/(...)(Fii )(...)/;
$string =~ s/$regex/$1 . changer("$2") . $3/e;
ok($string eq 'feefiifoo', 'change field 2');

sub changer {
    my $str = $_[0];
    $str =~ s/\s//g;
    return lc $str;
}

##</code><code>##

#Untried
$_ =~ s/$regex_2/$+{group_x} . $+{group_y} . changer("$+{text}")/e;