I'd tried negative lookahead before without success. But your suggestion made me try harder and I finally got it to work! I had a problem with the order of the characters in $junk being strangely significant until I switched to single quotes:

my $junk   = '<>]="\'';

$text =~ s{(?!$junk)\b($urls:[$any] +?)(?=[$punc]* [^$any]|$) }{<a hre
+f="$1">$1</a>}migox;
[download]

/me hugs suaveant