If I understand this correctly, you don't need to do anything special to preserve the order, because the order can be reconstructed using a custom sort routine.

For example:

my %pres = ();
$pres{ s1 }{ Name } = 'name of first presentation';
$pres{ s1 }{ Grade } = 5;
#etc.
for my $id ( sort ById keys %pres ) {
    # whatever
}

sub ById {
    GetNumeric( $a ) <=> GetNumeric( $b );
}

sub GetNumeric{
    my $subject = shift;
    /(\d+)/;
    return $1;
}
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