#!/usr/bin/perl --
use warnings;
use strict;
use feature qw{ say };
use File::Which qw/ which /;
our $GOTPS = which('ps');

say $$;
psMem();

my $file = shift;
open my $FH, '<', $file or die $!;

my $string;

{ local $/; $string = <$FH> };
say length $string;

psMem(); 

undef $string;
psMem(); 

seek $FH, 0, 0;
$string = do { local $/; <$FH> };
say length $string;

psMem();
undef $string;
psMem(); 


sub psMem {
    my( $pid ) = @_;
    my @lines = qx{$GOTPS   -o vsize,rss     -p $pid };
    chomp @lines;
    foreach my $line (@lines) {
        my ( $vsize, $rss) = split /\s+/, $line;
        return "VM: $vsize   RSS: $rss  ";
    }
    return;
}