Refactoring this into two different classes would be creating more duplication of code. since the rest of logic is the same.(handling missing field, and what not..;)

So make 3 classes , they don't have to OO :) MyRecs; MyRecs::Big; MyRecs::Lil;

sub MyRecs::foo {
    my( $ro, $sham, $bo ) = @_;
    die "sham must sham " if not $sham =~ /sham/i;
    ...
}
sub MyRecs::Lil::foo {
    MyRecs::foo( unpack "n3A*", ... );
}
sub MyRecs::Big::foo {
    MyRecs::foo( unpack "v3A*", ... );
}
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