http://www.perlmonks.org?node_id=11119555

The expression \$alice eq \$bob is returning true because both \$alice and \$bob are undef, that's both contain the same information hence they are equal. Am I right?
• Comment on Re^3: Learning Programming, desperately need to know what information is contained in scalar variables

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Re^4: Learning Programming, desperately need to know what information is contained in scalar variables
by davido (Cardinal) on Jul 20, 2020 at 17:36 UTC

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

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.

Dave