Much too complicated. Why generate a random character and then use a regex to delete it? Even Arien's good suggestion can be written briefer.

my @pass_char = ('a'..'z', 'A'..'Z', 0 .. 9, qw(_ . @));
sub generate_password {
    my $len = shift || 12;
    my $pass;
    $pass .= $pass_char[rand @pass_char] for 1 .. $len;
    return $pass;
}
[download]

I can't resist golfing this a bit.. but the following is not necessarily for use in production. :-)

my @pass_char = ('a'..'z', 'A'..'Z', 0 .. 9, qw(_ . @));
sub generate_password {
    return join '', map $pass_char[rand @pass_char], 1 .. (shift || 12
+);
}
[download]

____________
Makeshifts last the longest.