HTML::Entities is a good idea. here is a version that doesn´t need a regex:

use HTML::Entities;

my $str = q("I have a 15" latter and a 6" foot arm.");
substr($str, 1, length($str)-2) = encode_entities(substr($str, 1, leng
+th($str)-2));

print $str; #"I have a 15" latter and a 6" foot arm."
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