If the type of a value isn't the type called for, Perl automatically converts the value to the appropriate type. undef gets converted to zero when a number is expected, such as for the operand of the numerical equality operator (==).

use strict;
use warnings;

my $undef;
my $non_numeric_str = 'abc';

print "it's 0\n" if $undef           == 0;
print "it's 0\n" if $non_numeric_str == 0;
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Use of uninitialized value in numeric eq (==) at script.pl line 7.
it's 0
Argument "abc" isn't numeric in numeric eq (==) at script.pl line 8.
it's 0
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