That doesn't look so inelegant to me, but here's a way that avoids the substitution, FWIW (though I'm sure someone can do better):

$string = 'garbage validcommand 1234  221 garbage 999 213';
$string =~ m/validcommand\s+((\d+\s+)+)/;
@args = split ' ',$1;
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I'd like to be able to assign to an luser