my $reg1 = 1;
my $reg2 = 0;
...

my @regex;

push(@regex,[qr/^unleserliches\s+Konstrukt/,sub{ ... }]) if ($reg1);
push(@regex,[qr/xyz/,sub{ ... }]) if ($reg2);
...

while (my $line=<$dat>){
  $line=~$_->[0] && $_->[1]->($line,...) for(@regex);
}