my $string = '"I have a 15" latter and a 6" foot arm."';
substr( $string, 1, -1 ) =~ s["]["]g;
print $string;

"I have a 15" latter and a 6" foot arm."

##</code><code>##

my $string = '"I have a 15" latter and a 6" foot arm."';
$string =~ s[(?<=.)"(?=.)][&quot;]g;
print $string;

"I have a 15&quot; latter and a 6&quot; foot arm."