My normal way for three required parameters is:

use Class::ParmList qw(simple_parms);

sub convert {
   my ($from, $to, $thing) = simple_parms([qw(from to thing)], @_);

   #...
}
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When I finish my Sub::Parms module that will become (using the default 'required' mode):

use Sub::Parms;

sub convert {
    BindParms : (
        my $from  := from;
        my $to    := to;
        my $thing := thing;
    )
    
    #...
}
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