my @a = (
'0 11 11 / 11 11',
 '0 11 11 / 111 111',
 '0123 / 111 11 11',
 '011 / 1 111 1111',
 '012 3 / 11 11 1 ',
);

sub formatieren {
        my $num = shift;
        $num =~ s/\s//g;
        $num = reverse join(' ', unpack('(A2)*', reverse $num));
        return $num;
}

for my $tel (@a) {
        $tel =~ s#^([\d ]+)/([\d ]+)$#formatieren($1).' / '.formatieren($2)#e;
        say $tel;
}