While matija has your bugfix, you could try the line-by-line approach:

open PS, 'ps -ef -o pid,ppid,pgid,user |' or die;
while (<PS>)
{
  tr/ //s;
  s/^ //;
  print;
}
close PS or die;
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but I think what you really want is split and join (or $" or $,). Some golfing gives:

#!/bin/sh
ps -ef -o pid,ppid,pgid,user | /usr/bin/perl -lape '$_="@F"'
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