texuser74,
If you take a look at perldoc perlvar you will find plenty of interesting variables to make you life easier. In this case, I used $/ - the input record seperator.

$/ = "<sep>\n";
while ( <DATA> ) {
    chomp;
    print "$_\n" for sort { $a <=> $b } split /\n/;
    print $/ if ! eof DATA;
}
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