Fork a child to filestat (unix stat is required or windows equivalent) while parent copies on...:

#!/usr/bin/env perl

use strict;
use warnings;

# assumes file src is 'aaa' and dest is 'bbb'

my $cmd1 = 'cp'; # path to OS filecopy command
my @args1 = ('aaa', 'bbb');
my $cmd2 = 'stat'; # path to OS file-stat command
my @args2 = qw/-c '%s' bbb/; # unix stat params to print size of targe
+t file 'bbb'

my $pid = fork();
die "fork! $!" if not defined $pid;
if( $pid > 0 ){
        # child
        my $keepon = 1;
        $SIG{TERM} = sub { $keepon = 0; print "sigterm received\n"; };
        sleep 2; # let the copy begin first ...
        while($keepon){
                open(EXE2, '-|', $cmd2, @args2) or die "$!";
                my $sta = '';
                while(<EXE2>){ $sta .= $_ }
                print "$0 : file size now is $sta\n";
                close(EXE2);
                sleep 1;
        }
        print "child out of loop and dead...\n";
} else {
        # parent
        print "$0 : starting copy ...\n";
        system($cmd1, @args1) == 0 or die "system failed $cmd1, $!";
        print "parent exiting\n";
        kill -15, $pid; # kill the child :(
}
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