in reply to Unusual Closure Behaviour

HI, Try this, Here it says that $x is not defined and therefore it creates a new local variable $x with 'my'.
Otherwise it uses the last value of x which is the default behaviour.
sub foo { my $x if !defined $x; return ++$x; } for (1..7){ print foo(); };

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(tye)Re2: Unusual Closure Behaviour
by tye (Sage) on Jul 13, 2001 at 03:20 UTC

    Actually, that code isn't doing what you think. One way to see is to just add "use strict" to it. You'll get: Global symbol "$x" requires explicit package name

    A more fun way is:

    sub foo { my $x= $_[0] if ! defined $x; print "(@_):", ++$x, " "; undef $x if @_ && shift; } foo($_) for( 0,0,5,4,3,0,5,0,4,0,3 ); print $/, $x= 10, $/; foo($_) for( 0,0,5,4,3,0,5,0,4,0,3 ); print $/;
    which produces:
    (0):1 (0):1 (5):6 (4):5 (3):4 (0):1 (5):6 (0):1 (4):5 (0):1 (3):4 10 (0):1 (0):2 (5):3 (4):1 (3):1 (0):1 (5):2 (0):1 (4):2 (0):1 (3):2
    The $x in "if !defined $x" is the global $main::x because my variables can't be used until after the end of the statement in which they were declared.

    So your code is always initializing the lexical $x in your subroutine because you have never defined the global $main::x that you are checking against.

            - tye (but my friends call me "Tye")