There is no ASCII character A0 or B1. If the data is arbitrary bytes, then the following will do:

pack('H*', '4142A0B1')
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If the data is actually text (as implied by "ASCII"), you'll need to determine the actual encoding used and use

decode($enc, pack('H*', '4142A0B1'))
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(The decode step can be skipped for iso-8859-1.)

For example,

use strict;
use warnings;

use open ':std', ':locale';

use Encode qw( decode );

for my $enc (qw(
    ASCII
    cp850
    cp1252
    iso-8859-1
    UTF-8
)) {
    my $s = decode($enc, pack('H*', '4142A0B1'), sub { "?" });
    printf("%-11s %s\n", "$enc:", $s);
}
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ASCII:      AB??
cp850:      ABá▒   Common "western" code page for Windows console
cp1252:     AB ±   Common "western" code page for Windows gui apps
iso-8859-1: AB ±   Common "western" unix encoding
UTF-8:      AB??

Note that cp1252 and iso-8859-1 are not equivalent despite producing the same output in this situation.