Perl's operators are monomorphic, and its data is polymorphic. Look at this:

print "Yes\n" if 1 == "1"; # Yes
print "Yes\n" if "0" eq 0; # Yes
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In the first test, the number 1 is compared numerically to the string "1". Perl converts that string to a number using rules defined in perldata, and perlop.

In the second test, the string "0" is compared to the number 0 using stringwise comparison. So the number 0 is promoted to a string for the comparison.

If you compare undef to undef numerically, undef is treated as 0. If you compare undef to undef stringwise, undef is treated as empty string. So undef == undef produces the same result (true) as 0 == 0. And undef eq undef produces the same value (a Boolean true value) as '' eq '' (empty string is equal to empty string).

To put it even more plainly:

• '' eq '' # true: Empty string is equal to empty string.
• undef, when treated as a string acts like an empty string.
• The eq operator treats its operands like strings.
• If $foo and $bar are undef, then $foo eq $foo becomes an empty string to empty string comparison.