Replace the string/line after separator on specific param

I have the following Perl one liner code ,

My target is to replace the line after “=” separator, and that defined in PARAM variable

more file.txt
PARAM=I WANT TO REPLACE THIS LINE

bash-3.00# export VAL="THIS IS THE NEW LINE FOR EXAMPLE"
bash-3.00# perl -i -pe 's/(PARAM=)+/${*}$ENV{VAL}/' file.txt
bash-3.00# more file.txt
THIS IS THE NEW LINE FOR EXAMPLEI WANT TO REPLACE THIS LINE

But actually its replace all the line include the PARAM itself

The expected results should be

more file.txt
PARAM= THIS IS THE NEW LINE FOR EXAMPLE

Please advice what need to fix in my Perl one liner code?

other example that still not work:

bash-3.00# more file.txt
PARAM=SOME STRING
bash-3.00# export VAL="PUT THAT"
bash-3.00# perl -i -pe 's/PARAM=K.+/$ENV{VAL}/' file.txt
bash-3.00# more file.txt
PARAM=SOME STRING

Answer

You can use K (positive look behind) and .+ to remove everything after PARAM= until newline,

perl -i -pe 's/PARAM=K.+/ $ENV{VAL}/' file.txt

or for older perl,

perl -i -pe 's/(?<=PARAM=).+/ $ENV{VAL}/' file.txt