Folks have already pointed out the weirdness result of mixing my and if in the same statement, but I wanted to point out another stylistic issue. You define $func as a subroutine reference whether it's provided by the user or defined by wannabe_foreach, but you've got it going in two distinct places. Why not mush them together so folks can easily see what $func's ultimate fate is?

use warnings;
use strict;

# A 'wannabe' foreach, which:
#   - takes a CODE ref as an optional first argument
#   - applies it to the rest of the args
#   - If no CODE ref was passed, then it uses a closure of its
#     own that simply shoves all the args into an accumulator list
#     which it just dumps to STDERR
sub wannabe_foreach {
    my ($func, @list, @accumulator);

    # Use provided code-ref if there is one,
    # otherwise create a simple subref to push onto @accumulator
    if ( ref $_[0] eq 'CODE' ) {
        $func = shift; # optional code-ref
    }
    else {
        $func = sub { push @accumulator, shift };
    }

    # If you absolutely gotta be terse there's always
    # $func = ( ref $_[0] eq 'CODE' )
    #       ? shift
    #       : sub { push @accumulator, shift }
    #       ;

    @list = @_;

    $func->($_) for @list;

    warn join ",", @accumulator if @accumulator;

}

wannabe_foreach(1..3); # Works. Warns: '1,2,3 at foo.pl line ...'
wannabe_foreach(1..4); # No funkiness here
[download]