I'm thinking you don't need a beefy regex if that's all you want to do. How about something like this instead:

my $string = '12.34.56.78 asdfasdfasdfasdfasdf';

my $numbers = shift @{[split ' ', $string]};
my @numbers = split '\.', $numbers;

use Data::Dumper;
print Dumper(@numbers) if $numbers =~ /^[\d.]+$/;
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EDIT: Whoops, sorry moritz - I confess to only skimming your answer and didn't notice that you had already suggested using split. Apologies!