You have it backwards, as far as I'm concerned. Your snippet would still fail without wantarray. Without wantarray, my $scalar = $cgi->param('foo'); would *also* fail.

That's the case for every function that returns a list. Perl can't return a list in scalar context, so param must necessarily return something different in scalar context. wantarray simply helps return a meaningful value instead of junk.

Since param returns a list, feel free to always call param in list context if you want to avoid the issue.

my @values = $cgi->param('foo');

my ($first_value) = $cgi->param('foo');

my $first_value = ( $cgi->param('foo') )[0];
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"would *also* fail" - um, no it wouldn't, because it explicitly returns a scalar undef. I've tested it.

Of course, what it would do is change an array assignment:

# from
@array=();

# to:
@array=(undef);
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Which would be an unacceptable change to existing behavior (exists $array[0] value would change from empty string to 1).

Not that situation, the situation where the field is present.