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;
}