in reply to How to use 2 files for calculating charges

This one may be over my head: 
C:\Steve\Dev\PerlMonks\P-2013-08-13@1518-Time-Days-Discount-Computatio
+n>doit1.pl
Possible unintended interpolation of @ymd2 in string at C:\Steve\Dev\P
+erlMonks\P-2013-08-13@1518-Time-Days-Discount-Computation\doit1.pl li
+ne 33.

Line 33:    my @ymd1 = split ',',$dates[4] //= $prev_mth_end;

Execution of C:\Steve\Dev\PerlMonks\P-2013-08-13@1518-Time-Days-Discou
+nt-Computation\doit1.pl aborted due to compilation errors.

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So my question is, what the heck does  //=  do?

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Re^2: How to use 2 files for calculating charges
by Laurent_R (Canon) on Aug 13, 2013 at 22:02 UTC

    So my question is, what the heck does //= do?

    This is the relatively new "defined or" operator

    $a //= $b;

    means that $a is unchanged if it is defined, and is assigned to $b if not defined.

    At least, that's my understanding, I have never used it, but the following test under the Perl debugger seems to confirm it: 

    $ perl -de 42

Loading DB routines from perl5db.pl version 1.33
Editor support available.

Enter h or `h h' for help, or `man perldebug' for more help.

main::(-e:1):   42
  DB<1> $a //=5;

  DB<2> p $a
5
  DB<3> $a //=6;

  DB<4> p $a
5
  DB<5>

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      This feature added in perl v5.10.0 Refer perl5100delta:
      and the statement $c //= $d; can now be used instead of $c = $d unless defined $c;
      Bill
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