variable_check.pl

#!/usr/bin/env perl
use strict;
use warnings;

# こんな変数がありましたとさ
my $data = { param => "foo\n" };

for (0..1) {
    # 丁寧に？書くとこうなる
#   if ($data->{param}) {
    if (defined($data->{param})) {
        print $data->{param};
    } else {
        print '$data->{param}' . " is nothing.\n";
    }

    $data->{param} = undef;
}

$data->{param} = "baz\n";
for (0..1) {
    # 1 line な感じで書くとこうなる
#   if ($_ = $data->{param}) {} else { $_ = '$data->{param}' . " is nothing.\n"; } print;
    if (defined($_ = $data->{param})) {} else { $_ = '$data->{param}' . " is nothing.\n"; } print;

    $data->{param} = undef;
}

$data->{param} = "bar\n";
for (0..1) {
    # さらに, 存在しない時に出力しなくていいならば
#   print if $_ = $data->{param};
    print if defined($_ = $data->{param});

    $data->{param} = undef;
}