s/this/that/
s#this#that#
s#this# "th" . "at" #e   <-- the /e means (e)valuate the RHS before doing substitution

# so the LHS of our RE is just this
^        begin string
(        begin capture into $1
\d{16}   16 digits
)        end capture into $1


# when we get a match of 16 digits at the begining of the string then we have 2 events. 
# first $1 contains them and 
# second the RHS of the RE gets evaluated.

$_ = 1;      # set a var to $1 so we can modify it (can't do to $1 as read only)
tr/0-9/A-J/; # transliterate contents of $_ aka $1
$_;          # perl will eval this with the net result that 
             # our 'that' result is $_ which duly gets used
             # to replace our original digits