G'day Bod,

"... reads JSON data from STDIN on a webserver. ... I have had to simulate this."

There's a number of ways a program can read from STDIN:

# By default
$ cat > TabNL
Tab     NL

# By redirection
$ cat -vet < TabNL
Tab^INL$

# By piping
$ cat TabNL | cat -vet
Tab^INL$

# By using '-' as a special filename
$ cat -vet -
Tab     NL
Tab^INL$

# By others I didn't immediately think of
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Which of those methods does your module use? Knowing this will allow us to better advise you on ways to perform the simulation. :-)

"Using code I found in an answer on SO ... *STDIN = *DATA; ..."

That's not really simulating STDIN. You're just rebadging an existing filehandle:

$ perl -E '
use strict;
use warnings;
say "Real \\*STDIN fileno: ", fileno(\*STDIN);
say "Real \\*DATA  fileno: ", fileno(\*DATA);
*STDIN = *DATA;
say "Fake \\*STDIN fileno: ", fileno(\*STDIN);
__DATA__
some data
'
Real \*STDIN fileno: 0
Real \*DATA  fileno: 3
Fake \*STDIN fileno: 3
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Beyond simulating the input, it would probably help to have some idea of what tests you intend to run.

Here's a test script that simulates JSON being piped to your application. It's subsequently decoded and compared with reference data via is_deeply() (presumably you'd have more useful tests here). Note how you can run the tests on multiple JSON files.

ken@titan ~/tmp/pm_11152777_test_stdin/t
$ cat test_json.t
#!perl

use strict;
use warnings;
use autodie;

use Cwd 'abs_path';
use File::Basename 'dirname';
my $THISDIR;
BEGIN { $THISDIR = dirname abs_path __FILE__ }

use JSON::MaybeXS;
use POSIX '_exit';
use Test::More;

my @file_bases = qw{test1 testA};
plan tests => 0+@file_bases;

for my $file_base (@file_bases) {
    my $json_data = '';
    my $child_pid = open my $from_kid, '-|';

    if ($child_pid) {
        # parent process (pipe from child):
        #   reads JSON from "effective" STDIN
        while (my $line = <$from_kid>) {
            $json_data .= $line;
        }

        waitpid $child_pid, 0;
    }
    else {
        # child process (pipe to parent):
        #   writes JSON to STDOUT
        my $json_file = "$THISDIR/../data/$file_base.json";
        open my $json_fh, '<', $json_file;

        while (my $line = <$json_fh>) {
            print $line;
        }

        _exit 0;
    }

    my $perl_data = decode_json($json_data);
    my $reference_data = do "$THISDIR/../data/$file_base.perl";
    is_deeply $perl_data, $reference_data, "Testing '$file_base'";
}
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Here's the test data:

ken@titan ~/tmp/pm_11152777_test_stdin/data
$ cat test1.json
{
    "key1" : "val1",
    "key2" : [
        "elem1",
        "elem2",
        "elem3"
    ],
    "key3" : {
        "name1" : "value1",
        "name2" : "value2"
    }
}

$ cat test1.perl
{
    key1 => 'val1',
    key2 => [qw{elem1 elem2 elem3}],
    key3 => {name1 => 'value1', name2 => 'value2'},
};

$ cat testA.json
{
    "keyA" : "valA",
    "keyB" : [
        "elemA",
        "elemB",
        "elemC"
    ],
    "keyC" : {
        "nameA" : "valueA",
        "nameB" : "valueB"
    }
}

$ cat testA.perl
{
    keyA => 'valA',
    keyB => [qw{elemA elemB elemC}],
    keyC => {nameA => 'valueA', nameB => 'valueB'},
};
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And here's an actual test run:

ken@titan ~/tmp/pm_11152777_test_stdin
$ prove -v t/test_json.t
t/test_json.t ..
1..2
ok 1 - Testing 'test1'
ok 2 - Testing 'testA'
ok
All tests successful.
Files=1, Tests=2,  1 wallclock secs ( 0.01 usr  0.03 sys +  0.12 cusr 
+ 0.08 csys =  0.25 CPU)
Result: PASS
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