Would Filesys::Df, or Filesys::DiskFree help you out here? A small working example I just whipped up (with a bit of "sauce" for commifying the output):

use strict;
use Filesys::Df;

my $fobj = df("/");
print commify($fobj->{bavail} * 1024) . " bytes of "
        . commify($fobj->{blocks} * 1024) . " bytes free\n";

sub commify {
        my $text = reverse $_[0];
        $text =~ s/(\d{3})(?=\d)(?!\d*\.)/$1,/g;
        return scalar reverse $text;
}
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That being said, I hope you're using open() or system() in list-mode, and not using backticks to get this info, if you're not using one of these modules. Something like:

use strict;

my $fs  = join(' ', @_);
open(DF, "df $filesystems |");
my @df  = <DF>;
close(DF);
chomp(@df);
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