# encode all the '#' chars before we join
s/#/%35/g for values % details;
# now the only '#' chars represent our delimiter
my $entry = join "#", values %details;

# getting the data from the file we reverse the process
# first split on the '#' char to get our values
my @values = split "#", <FILE>;
# now unencode any encoded '#' chars in @values
s/%35/#/g for @values;