Enter functional programming techniques.

sub by_hashval_func {
    my ( $hashref ) = @_;
    return sub { $hashref->{ $a } cmp $hashref->{ $b } };
}

foreach my $key ( sort by_hashval_func( \%hash ) keys %hash ) {
    # do something useful
}
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Update: I just remembered that $a and $b are package globals, ie the above will not work if by_hashval_func is in a different package than the sort call ultimately using the closure. To cater to that case, the prototyped sort block form is necessary:

sub by_hashval_func {
    my ( $hashref ) = @_;
    return sub($$) { $hashref->{ $_[0] } cmp $hashref->{ $_[1] } };
}
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