UPDATE: I answered my own question here. And, the moral of the story is: those that live by $_ will die by $_ -- or, at least, are in danger of quite a nasty cut, requiring stitches and everything.

I was footling around looking at IO layers. So I set up some strings containing various test patterns, and used open FH, '<', \$foo to read each one -- so I could see what happened to each under different conditions.

The code below illustrates the general shape of what I was doing.

The interesting thing is that when I run this (v5.10.0) I get:

Fits: 5, 6, 7
 Read: 'Fit I'
 Read: 'Fit II'
 Read: 'Fit III'
Fits: 5, 6, 7
 Read: 'Fit I'
 Read: 'Fit II'
 Read: 'Fit III'
Fits: undef, undef, undef
...so, somehow or other the entries in the array @fits have been set undef during the second foreach. Uh ?

The only difference between the first and second foreach is the use (or not) of a named variable. This is a serious surprise to me ! (I tried: our and my work the same way.)

Using the string as a "file" is obviously material. I haven't spotted any indication that this "eats" the string being "input". But, even if it does, my working model for Perl's parameter passing is that the my ($what) = @_ makes a copy of $_[0] (not another alias for whatever $_[0] is an alias for)...

So who mangled the entries in @fits, and was it with the lead pipe or the rope ?

Colour me curious -- any colour you like.

UPDATE: sorry ! The penny just dropped. It's a side effect of the file reading setting $_ to undef... and $_ is global, of course. Silly me. I'll get my coat.

use strict;
use warnings;

my $file_a = "Fit I" ;
my $file_b = "Fit II" ;
my $file_c = "Fit III" ;

my @fits = ($file_a, $file_b, $file_c) ;

show_fits() ;

foreach my $f (@fits) { suck_this($f) ; } ;

show_fits() ;

foreach       (@fits) { suck_this($_) ; } ;

show_fits() ;


sub show_fits {
  print "Fits: ",
        join(', ', map { defined($_) ? length($_) : 'undef' } @fits),
        "\n" ;
} ; 

sub suck_this {
  my ($what) = @_ ;

  open my $FH, '<', \$what ;

  print " Read:" ;
  while (<$FH>) {
    s/\r/\\r/g ;
    s/\n/\\n/g ;
    print " '$_'" ;
  } ;

  print "\n" ;
} ;

[download]

In reply to Just when you thought you'd got it: 'foreach $f (@foo)' vs 'foreach (@foo)' by gone2015

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.