I've been refactoring the Parrot build tool pmc2c.pl for a month-and-a-half now, and only last night did I realize how mysterious I find the following formulation: 

    my $fh = open_file("<", $file, $verbose);
    my $class;
    eval do { local $/; <$fh> };
    die $@ if $@;

    close $fh;
    return $class;

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$file, when opened (I suspect) looks like this: 

$class = {  big => "hashref" };

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When I surround this code with print statements for debugging, I find that $class is undefined when first declared but fully populated at the point where it is to be returned -- even though there is no explicit assignment to $class in this program.

Let me illustrate. Suppose that I have a file called alpha.txt that reads like this: 

$class = {
    alpha      => 'beta',
    gamma   => 'delta',
    epsilon   => 'zeta',
};

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In my main program, I then call: 

use strict;
use Data::Dumper;
my $file = q{alpha.txt};
my $class;
open my $fh, "<", $file or die "Unable to open: $!";
eval do { local $/; <$fh> };
die $@ if $@;
close $fh or die "Unable to close: $!";
print Dumper $class;

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I get this output: 

$VAR1 = {
          'gamma' => 'delta',
          'epsilon' => 'zeta',
          'alpha' => 'beta'
        };

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Now, I would not have been surprised to get this result if my code had read: 

use strict;
use Data::Dumper;
our $class;
my $file = q{alpha.txt};
require $file;
print Dumper $class;

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... which produces exactly the same output. But in this case I know I have to use an our variable; a my variable will not suffice. So how come eval do {} enables me to get away with a my variable in the former case?

Thank you very much.

Jim Keenan

In reply to eval do {} -- magical variable assignment? by jkeenan1

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