Here is one way, using tr and substr (without using regular expressions):

use warnings;
use strict;

while (<DATA>) {
    tr/-abcdef/9012345/;
    print substr($_, 0, 10);
    print "\n";
}

__DATA__
a81f9284-b353-430a-a8ce-85b8118d3f3e
[download]

See also (documentation available at your command line via perldoc):

• perldoc perlop
• perldoc perlre

To grab the output of an external command, such as uuidgen, you should use qx instead of system.