I don't think there is a general soultion that will work in all cases. There are just too many ways to use version numbers and put them in a filename.

You might be able to get the correct number much of the time with something like this:

# $filename declared elsewhere
$filename =~ /\A
    [^-]+       # Read until the first '-'
    ([\d\.]+)   # Grab a series digits and '.' chars
    /x;
my $version = $1;
[download]

But that will fail on many version numbering systems.

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Reinvent a rounder wheel.

Note: All code is untested, unless otherwise stated