The problematic statement is
    my $fmt = $odd_format{$register} // $default_format;
which uses the  // (defined-or) operator introduced with Perl version 5.10.

Because any value of  $odd_format{$register} (i.e., a format string) is highly unlikely to be false (see What is true and false in Perl?), it's almost certainly ok to use the  || (logical-or) operator instead (all the following alternatives are untested):
    my $fmt = $odd_format{$register} || $default_format;

An absolutely bullet-proof alternative uses exists and the  ? : (ternary) operator:
    my $fmt = exists($odd_format{$register}) ? $odd_format{$register} : $default_format;