Note: untested code follows

If you know that #s won't appear in your data, you can write:

$line =~ s/^([^#]*)#.*$/$1/;
[download]

If #s can appear in quoted strings, life gets a little more complex:

$line =~ s/^
           (           # grab this stuff in $1
             (
             [^#"]*    # prefix of non-#s, non-"s
             (\"       # start of string
               [^\"]*  # content of string
              \")?     # end of string
             [^#"]*    # suffix
             )*        # grab many prefix-string-suffixes
           )
           \#          # start of comment
           .*
           $
          /$1/x;
[download]

(At this point, you may be better off using one of the Text modules, and if the input's really hairy, Parse::RecDescent.)

Update: Er, that second regex is s/.../$1/x;, not s/.../x;. Doh!

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:wq