tr/// is insufficient because you want to match a letter followed by a number. I'm not sure if I've taken the best approach, but it's one way. 
#!/usr/bin/perl

use strict;
use warnings;

my $string = "A1 and A2 but not 1 and 2";

my $switch_hash = { 1 => 2, 2 => 1 };

my $regexp = '([A-Za-z])('.(join( '|', keys %$switch_hash )).')';

print "Regexp: $regexp\n";

my $comp_regexp = qr"$regexp";

$string =~ s/$comp_regexp/$1.$switch_hash->{$2}/ge;

print $string, "\n";

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In reply to Re: replace a digit with another digit by Transient
in thread replace a digit with another digit by kevyt

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