Shell Programming and Scripting

hi everybody!
i am running this ksh script for replacing a set of strings by another set of new ones. i am getting both these from a file.
also, the strings that i want to replace, are sub-strings(can occur more than once in each chunk) in a big chunk of data that i have bulk-copied(bcp utility) from a database. i can do it successfully, but it so happens that some of the set of strings that need to be replaced are mutually sub-strings of eachother. like for example, the file containing the sets of strings includes the following::

abc abc:x
abcd abcl.x
...etc.

now while replacement with the first set of strings the instances of the second string get altered...
abc:xd

i am trying to use gawk FIELDWIDTHS for this with the following code snippet:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cp ${ORIGFILE} ${TMPFILE}

sed "s/\*/\\\*/g" ${SYMLISTFILE} > ${NEWSYMLIST}
while read OLDSYM NEWSYM FLDLEN
do
gawk 'BEGIN{ FIELDWIDTHS = ${FLDLEN} } END{ s/${OLDSYM}/${NEWSYM}/g }'
# sed "s/${OLDSYM}/${NEWSYM}/g" ${TMPFILE} > ${INFILE}
cp ${INFILE} ${TMPFILE}
done < ${NEWSYMLIST}
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
while running the script now i get the following error for the ${FLDLEN}::
gawk: cmd. line:1: BEGIN{ FIELDWIDTHS = ${FLDLEN} } END{ s/${OLDSYM}/${NEWSYM}/g }
gawk: cmd. line:1: ^ parse error

please can anyone help me writing a viable code that can access the FLDLEN from the ksh script to the gawk script?
is gawk freely available to install... and is its installation the solution?

Thanking you in advance,
Tru.

Code:

 Fieldwidths = '$fldlen'