Below generates the same results w/o using the module:
use strict;
use warnings;
my @array1 = split /\n/, <<END1;
/home filer1232:/vol/home1
/mystuff filer1233:/vol/project
/data filer1234:/vol/example_data
/software filer1255:/vol/my_software
/tools filer1235:/vol/my_tools
/docs filer146:/vol/my_documents
END1
my @array2 = split /\n/, <<END2;
/home filer1232:/vol/home1
/mystuff filer1233:/vol/project
/data filer1234:/vol/example_data
/tools filer1235:/vol/my_tools
END2
my %currMounted = map { $_ => 1 } @array2;
print "$_\n" for grep !$currMounted{$_}, @array1;