How about:

  $$text_ref =~ s|(?<="http://)(.*?)(?=["/])|$fixers{$1} ? $fixers{$1}
+ : $1|ge ;
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which does one scan of the input. This may be an advantage if you have a lot of fixers. But, if there are a lot of things that match, but don't require fixing, doesn't work so well.

the more complicated:

  my $what = join('|', map(quotemeta, keys %fixers)) ;
  $$text_ref =~ s!(?<="http://)($what)(?=["/])!$fixers{$1}!ge ;
[download]