As an illustration, the following doesn't work cause &b is unknown when &a is compiled.

Without strict b will be converted to string "b" and returned. (strict prohibits this behaviour of barewords.)

lanx@nc10-ubuntu:/tmp$ perl
sub a { print "a$level "; b if $level-- };
sub b { print "b$level "; a if $level-- };
$level=5;
print"\n";
a;
print"\n";
b;
__END__

a5 
b4 a3
[download]

with forward declaration

lanx@nc10-ubuntu:/tmp$ perl
sub b;
sub a { print "a$level "; b if $level-- };
sub b { print "b$level "; a if $level-- };
$level=5;
print"\n";
a;
__END__

a5 b4 a3 b2 a1 b0
[download]