You can set the hash up at the beginning from multiple strings using map.

perl -e '%h = map{split /X/} q{fooXbar}, q{bishXbash};
print qq{$_ -> $h{$_}\n} for jeys %h;'
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produces

bish -> bash
foo -> bar
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If you have to add another key/value pair subsequently you will have to use intermediate variables to avoid the clobber.

Cheers,

JohnGG