You mentioned Determining memory usage of a process..., which lists a second answer (for Linux). This would work without installing additional modules. You simply have to find out wether your ps has different options.

END {
    system qq(tasklist /v /fo LIST /fi "PID eq $$")
}
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END {
    system qq(wmic  PROCESS where "processid=$$" get * /FORMAT:LIST)
}
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#!/usr/bin/perl
use warnings;
use strict;
use IPC::Open3;

my $pid = shift || $$;
my $pid1 = open3(\*WRITE,\*READ,0,"/bin/sh");
            #if \*ERROR is false, STDERR is sent to STDOUT 


while(1){
 print WRITE "ps -o rss= -p $pid\n";
   select(undef,undef,undef,.1); # a small delay
    my $size = <READ>;
    print "$size";
}
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Reading from /proc/$pid/stat is also pretty easy, and while there are probably some portability concerns, it is at least the same across centos and ubuntu, which are where I spend most of my time.

vsize is part of the contents of the file.