I fail to see how a regex would figure out whether a value is larger or smaller than 3.

You can get clever with match-time pattern interpolation. For instance:

my @list = (1..5);
my $pattern = qr/\d+(??{if($& < 3) { "" } else { "(?<!\\d)" } })/;

my @res = grep /$pattern/, @list;
say join ", ", @res;
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This also (ab)uses negative look-behind assertions to create a subpattern that is guaranteed to fail (in this regex, not in general).

But I agree, subroutine references are the way to go here.