somefilter <somefile >somefile

somefilter <somefile | sponge somefile

content=$(cat somefile; echo a)
content=${content%a}

printf %s "$content" | somefilter >somefile

perl -MTie::File -e '
    tie @a,"Tie::File","your_file_here";
    # Do something...
'

$ ls -ld read_only/
dr-xr-xr-x 2 terdon terdon 4096 Apr 13 02:16 read_only/
$ ls -l read_only/file
-rw-r--r-- 1 terdon terdon 3 Apr 13 02:16 read_only/file
$ sed -i 's/a/A/' read_only/file
sed: couldn't open temporary file read_only/sedOEdv8L: Permission denied
$ perl -i -pe 's/a/A/' read_only/file
Can't remove read_only/file: Permission denied, skipping file.

$ perl -MTie::File -e 'tie @a,"Tie::File","$ARGV[0]"; $a[0]=~s/a/A/' read_only/file