Replace fixed pattern in file

Perl

perl -pi.bak -e '$from='\''fixed string'\''; $to='\''replacement string'\''; s/\Q$from\E/$to/g' <file>
 
# or
 
perl -s -pi.bak -e 's/\Q$from\E/$to/g' -- -from='fixed string' -to='replacement string' files/mini_snmpd.init

Even though perldoc perlrun says -s parsing is made before an argument of -- or any filename arguments, in practice a -- is needed before the arguments for this case.