use strict;
use warnings;
my $string = "aabbzzb";
my $oldchars = "ab";
my $newchars = "xx";
$_ = $string;
#replace the chars by "x" char.
eval "tr/$oldchars/$newchars/, 1" or die $@;
$string = $_;
#then delete the 'x' char.
$string =~ tr/x//d;
print $string; #will print zz