Since you want "1234 221", not "1234", which is what that regex will capture, and you really want to be using match, not substitute, I'd suggest this:

#!/usr/bin/perl -w 
use strict; 
 
my @args; 
my $string="garbage validcommand 1234  221 garbage 999 213"; 
while ($string =~ m!validcommand\s+(\d+(?:\s+\d+)*)!g) { 
  push(@args, $1); 
} 
print @args;
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