Backticks `` are used to get the results of a shell command: 
1042% df -k /dev/hda7
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/hda7              9606556   7156344   1962216  79% /
1043% perl -e'print `df -k /dev/hda7`'
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/hda7              9606556   7156344   1962216  79% /

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Update I forgot to answer the pure perl part of the question. The module Filesys::Df should be able to do what you want. From the synopsis: 
use Filesys::Df;

my $ref = df("/tmp");  # Default block size of 1024 bytes.
                           # You can specify a different block
                               # as a second argument.

print"Percent Full:       $ref->{per}\n";
print"Total Blocks:       $ref->{blocks}\n";
print"Blocks Available:   $ref->{bavail}\n";
print"Blocks Used:        $ref->{used}\n";
print"Inode Percent full: $ref->{fper}\n";
print"Inodes Free:        $ref->{favail}\n";
print"Inodes Used:        $ref->{fused}\n";

print"Total Bytes", ($ref->{blocks} * 1024), "\n";

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-Mark

In reply to Re: Calculating Mount Point Capacity by kvale
in thread Calculating Mount Point Capacity by St. Catharine

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