.gitignore

str.pl
undef.pl

Makefile

default : test

expand : undef.pl str.pl

undef.pl : no.pl
	perl -pe 's/my \$$res;/my \$$res = undef;/' no.pl > undef.pl
	chmod +x undef.pl

str.pl : no.pl
	perl -pe 's/my \$$res;/my \$$res = '"''"';/' no.pl > str.pl
	chmod +x str.pl

test : expand
	prove ./test.pl

no.pl

#!/usr/bin/env perl

sub test {
    local $SIG{ALRM} = sub {
        die;
    };
    alarm 2;
    my $res;
    eval {
        chomp($res = <STDIN>);
    };
    alarm 0;
    return $res;
}

print "1st : ", test(), "\n";
print "2nd : ", test(), "\n";

str.pl

#!/usr/bin/env perl

sub test {
    local $SIG{ALRM} = sub {
        die;
    };
    alarm 2;
    my $res = '';
    eval {
        chomp($res = <STDIN>);
    };
    alarm 0;
    return $res;
}

print "1st : ", test(), "\n";
print "2nd : ", test(), "\n";

test.pl

#!/usr/bin/env perl

use strict;
use warnings;

use Test::More 'no_plan';

my $expect = <<EOL;
1st : aa
2nd : 
EOL

foreach my $base (qw(no undef str)) {
    is(`( echo aa; sleep 3; echo bb ) | ./$base.pl`, $expect, "test for $base");
}

undef.pl

#!/usr/bin/env perl

sub test {
    local $SIG{ALRM} = sub {
        die;
    };
    alarm 2;
    my $res = undef;
    eval {
        chomp($res = <STDIN>);
    };
    alarm 0;
    return $res;
}

print "1st : ", test(), "\n";
print "2nd : ", test(), "\n";