If you're dealing with single characters, you can use tr:

tr/abcd/1234/;
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Otherwise a more complex way of doing that:

my %tr = (
  a => 1,
  b => 2,
  c => 3,
  d => 4
);

my $tr_keys = join('\E|\Q', keys %tr);

s/(\Q$tr_keys\E)/$tr{$1}/g;
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The latter method has the benefit of being able to work with arbitrarily long strings instead of just single characters.