It stops after the first match was found. So i get 1 entry in 1 File, and the rest of the file remains empty.

Are all the output files created, but empty? Are you sure that the one entry is correct? If your input is not broken into blocks correctly, the value of $/ is not correct. The scheme will not work if every end-of-block is not exactly the same. (Remember: $/ is a string) Please post a few (three to ten) blocks of realistic data. Use code tags so we can download it exactly. For security, you can use made-up data, but the format must be exact.

yes they are all created and empty.

the single match that is found is only found if the first block is a match. else everything is empty.

data looks like that:

UT 123456789
1234
9876
1234
some additional string information
THE_END
UT 987654321
1234
2345
some additional string information
THE_END
UT 1928374756
4321
2567
1234
THE_END
some additional string information
UT 5647382910
1234
2435
5678
some additional string information
THE_END
[download]

notice I changed END to THE_END to make it more unique, since other lines may accidentially contain the string "END" and I cant use regex "^END"

the current code is:

#!perl

use strict;
use warnings;
use FindBin;

my $dir ="$FindBin::Bin/../rxo";
opendir(my $dh, $dir) || die "can't opendir $dir: $!"; 
my @inputs = readdir($dh);
closedir $dh;
splice @inputs, 0, 2;

my @dispatch;
foreach(@inputs) {
    my $outfile = "$FindBin::Bin/../blocks/$_";
    open my $ofh, '>', $outfile || die; 
    my $file = "$FindBin::Bin/../rxo/$_";
    open my $fh, '<', $file || die;;
    my $regex = <$fh>;
    close $fh;
    push @dispatch, { file => $ofh, regex => qr/$regex/ };
    }

while(my $line = do { local $/ = 'THE_END'; <> }) {
   foreach (@dispatch) {
        print { $_->{file} } $line if $line =~ $_->{regex};
    }
}
[download]

Use $/ = "THE_END\n"; Your regex's are failing because every block but the first has a newline at the beginning. I should have thought of that sooner.
Bill