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

my $x;
my $y;

print $x - $y;

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Use of uninitialized value $y in subtraction (-) at warnings.pl line 8.
Use of uninitialized value $x in subtraction (-) at warnings.pl line 8.
0

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

my $x;
my $y = '1e';

print $x - $y;

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Argument "1e" isn't numeric in subtraction (-) at warnings.pl line 8.
Use of uninitialized value $x in subtraction (-) at warnings.pl line 8.
-1

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#!/usr/bin/env perl
use strict;
use warnings;
use Scalar::Util qw(looks_like_number);

my $x;
while (1) {
  print "数値を入力してください >>> ";
  $x = <STDIN>;
  chomp $x;
  last if want_number($x);
}
print qq{入力された数値は `$x` です};
exit;

sub want_number {
  my ($args) = @_;
  return 1 if looks_like_number($args);
  print "エラー：数値ではありません！\n";
  return;
}