I try this script, but it says "awk: fatal: can't open source file `BEGIN {"

script:
#!/bin/sh
DATA1=file1.txt
DATA2=file2.txt

awk -f 'BEGIN {
while ((getline < "'$DATA2'") > 0)
f2array[$2] = $1
OFS=","}

{if (f2array[$1])
print f2array[$1],$2,$3,$4,$5
}' $DATA1

Thanks for your time to read my thread Franklin