Rather than shifting @mylist and @mylist1, you can immediately shift @vmstat to get rid of the first record: saves you one shift operation and one loop through @vmstat.

It is not necessary to chomp $z when looping through @mylist and @mylist1 if you do not add the "\n" to the result of the split when looping through @vmstat: saves you two chomp operations.

You can even leave out the first chomp $x, as your split / /, $x will drop the EOL-character already.

Speaking of the split / /, $x: as you only need the fifth and sixth element of each record, why not saying:

(undef, undef, undef, undef, my $pos5, my $pos6) = split / /, $x

And finally (at the risk of becoming obfuscated):

foreach (@vmstat) { # assigns to $_
         (undef,undef,undef,undef,$pos5,$pos6) = split; # splits $_ on
+ whitespace
         push @mylist, $pos5;
         push @mylist1, $pos6;
}
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CountZero

"If you have four groups working on a compiler, you'll get a 4-pass compiler." - Conway's Law