But what is being evaluated by the regex? just the first element of the @_ array ?

The regex match is doing just what is in the code - matching the first element of @_ with the regex. The effect of the list context is that whatever is matched is returned e.g

print map("[$_]", "foo bar baz quux" =~ /b\w+ /g), $/;

__output__

[bar ][baz ]
[download]

This is the result of the g modifier in list context without capturing parens. With capturing parens however it returns a list of what was captured e.g

print map("[$_]", "foo bar baz quux" =~ /(b\w+) /g), $/;

__output__

[bar][baz]
[download]

For more info regarding regex matching and context see the Regexp Quote-Like Operators section in perlop.
HTH

_________
broquaint