Ok, I warmed up to this solution, thanks oshalla!

# pre build a pattern  "foo\.edu|fum\.edu"
my $what = join('|', map(quotemeta, keys %main::fixers)) ;
sub tweak_links($)
{
    my $text_ref = shift;
    return($$text_ref =~
     s!(?<="http://)($what)(?=["/])!$main::fixers{$1}!ge);
}
[download]

The overall script runtime dropped in half (0m4.8s vs. 0m7.2s on a test directory).