Your original code with strict and warnings: 

use strict;
use warnings;
sub twice { 2 * $_[0] }
my @a = map twice, 1..3;
print "@a\n";

[download]
gives the warning "Use of uninitialized value $_[0] in multiplication (*) at map1.pl line 3".

This is how it should be written: 

use strict;
use warnings;
sub twice { 2 * $_[0] }
my @a = map twice($_), 1..3;
print "@a\n";

[download]
which produces 2 4 6 as expected.

BTW, Conway, in Perl Best Practices, chapter 8, recommends always using the block form of map, not the expression form that you are using. The expression form might be faster than the block form though (if that matters, you should benchmark it).

In reply to Re^3: Problems with map(function, array) by eyepopslikeamosquito
in thread Problems with map(function, array) by Redei

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