use warnings;
use strict;

$| = 1;

my @categories = (
    'V=aeiou',
    'P=pbtdkg'
);

while (1) {
    print "Text: ";
    my $text = <STDIN>;
    chomp($text);
    last if ! $text;    # Leere Eingabe beendet Schleife

    print "Regel: ";
    my $rule = <STDIN>;
    chomp($rule);

    last if ! $rule;

    my ($search, $replace) = build_regex($rule);

    $text =~ s/$search/$replace/g;

    print "\nMuster: $search\n\nNach Ersetzung: $text\n\n";
}

sub build_regex {   # build_regex($rule);
    my ($to_replace, $replace_with, $pattern) = split('/', $_[0]);

    for my $category (@categories) {
        my ($type, $class) = split('=', $category);

        $pattern =~ s/$type/[$class]/g;
    }

    $pattern =~ s/(.+)_/(?<=$1)_/;
    $pattern =~ s/_(.+)/_(?=$1)/;
    $pattern =~ s/_/$to_replace/;    # <--- edit

    return ($pattern, $replace_with);
}
__END__

Text: akrta
Regel: k/g/V_rP

Muster: (?<=[aeiou])k(?=r[pbtdkg])

Nach Ersetzung: agrta