$_ = 'Hello World';

my %h = ( e => 'b', l => 'd', );

my $k = join '|', keys %h;

s/([$k])/$h{$1} || $1/eg;
print
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I think you want either ($k) or my $k = join '', keys %h; in the latter case one would probably also want to check that the keys are actually single letters. In both cases || $1 wouldn't be necessary any more, since the match would necessarily be one of the keys.