Setting aside why you want to do this, how about:

#!/usr/bin/env perl
use strict;
sub f {
        return 2;
}

sub g {
        my ($y) = shift @_;
        return $y*2;
}

if ( my $k = g(my $x = f())  ) {
        print "If the answer is four I'm happy.  Answer == $k\n";
}
[download]

That example seems to do all that you want.
update: Test platform was perl v5.8.6