Using this script:

#!/usr/bin/perl

use strict;
use diagnostics;
use warnings;

my $pid = fork || exec 'play', '/home/ra/Desktop/trek2.mp3';
print "My PID is *$pid*\n";
sleep 5;
kill $pid;
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The file starts playing, and I would hope that it stops playing after 5 seconds, but it doesn't. Here's the terminal output before the 5 second cut-off:

ra@eagle ~/Local/Code/amudclient/test/playsound $ ./playsound.pl
My PID is *21417*

/home/ra/Desktop/trek2.mp3:

 File Size: 506k      Bit Rate: 135k
  Encoding: MPEG audio    Info: 2017-09-26T15:54
  Channels: 2 @ 16-bit   Track: 309
Samplerate: 44100Hz      Album: This Week in Trek
Replaygain: off         Artist: Michael Gaines and Darrell Skeels
  Duration: 00:00:30.07  Title: This Week in Trek Episode 309

In:17.6% 00:00:05.29 [00:00:24.77] Out:233k  [     =|=     ]        Cl
+ip:0
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And the terminal output, once the file stops playing naturally.

ra@eagle ~/Local/Code/amudclient/test/playsound $ ./playsound.pl
My PID is *21364*

/home/ra/Desktop/trek2.mp3:

 File Size: 506k      Bit Rate: 135k
  Encoding: MPEG audio    Info: 2017-09-26T15:54
  Channels: 2 @ 16-bit   Track: 309
Samplerate: 44100Hz      Album: This Week in Trek
Replaygain: off         Artist: Michael Gaines and Darrell Skeels
  Duration: 00:00:30.07  Title: This Week in Trek Episode 309

In:17.0% 00:00:05.11 [00:00:24.96] Out:225k  [ -====|====- ]        Cl
+ip:0    raIn:28In:In:In:29.7%In:3In:3In:In:In:32In:InInIn:46.6% 00In:
+47.3% 0In:47InIn:100%  00:00:30.07 [00:00:00.00] Out:1.33M [   ===|==
+=   ]        Clip:0     
Done.
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kill $pid;

The first argument to kill should be the signal, not the PID. Since you didn't test the return value of kill you haven't spotted the error.