Here is a demonstration of it working, and some alternatives :) 

#!/usr/bin/perl --
use strict; use warnings;
use Data::Dump;
use JSON::XS;

my $ref = { foo => q{[1,2,3]} };
for my $key( qw/ bar foo /) {
    {
        my $foo;
        if( $ref->{$key} ){
            $foo = decode_json( $ref->{$key} );
        } else {
            $foo = [];
        }
        dd $foo;
    }

    {
        my $foo = [];
        if( $ref->{$key} ){
            $foo = decode_json( $ref->{$key} );
        }
        dd $foo;
    }

    {
        my $foo = $ref->{$key}
                  ? decode_json( $ref->{$key} )
                  : [];
        dd $foo;
    }

    {
        my $foo = eval { decode_json( $ref->{$key} ) } || [];
        dd $foo;
    }
}
__END__
[]
[]
[]
[]
[1, 2, 3]
[1, 2, 3]
[1, 2, 3]
[1, 2, 3]

[download]

In reply to Re^2: I can't see why this shorthand doesn't behave like the verbose form by Anonymous Monk
in thread I can't see why this shorthand doesn't behave like the verbose form by isync

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