#!/usr/bin/perl -w
# Note: the 'die' on the next line will probably never trigger
open PH, "ps -ef|" or die "$!: opening pipe";

# your snippet was missing this loop:
while (<PH>) {
  print;   # or whatever ...
}

# Note that for pipes, most errors will come when you try to close it, so
# if the command line had a problem, you'd see this 'die' trigger, not
# the one on the 'open'
close PH or die "$!: opening pipe";