Does Perl 6 have an equivalent to Python's bytearray method?

I think you're looking for Buf - a mutable sequence of (usually unsigned) integers. Opening a file with :bin returns a Buf.

brian d foy answer is essentially correct. You can pretty much translate this code into Perl6

 my $frame = Buf.new; 
 $frame.append(0xA2); 
 $frame.append(0x01); 
 say $frame; # OUTPUT: «Buf:0x<a2 01>␤»

However, the declaration is not the same:

bu = bytearray( 'þor', encoding='utf8',errors='replace')

in Python would be equivalent to this in Perl 6

my $bú =  Buf.new('þor'.encode('utf-8')); 
say $bú; # OUTPUT: «Buf:0x<c3 be 6f 72>␤» 

And to use something equivalent to the error transformation, the approach is different due to the way Perl 6 approaches Unicode normalization; you would probably have to use UTF8 Clean 8 encoding.

For most uses, however, I guess Buf, as indicated by brian d foy, is correct.