If so, try this recipe for in-place editing

prompt> perl -i -pe 's/"t/"GC_t/' file_to_be_changed.txt
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The -i flag tells perl to make changes (like regex substitutions) in the original file, dont make a backup

The -pe is two options joined together - -p is print every line in the supplied file back out to itself (thats what -i and -p together do - print every line back to the same file)

The -e tells perl to apply the regex to every line in the file - and hence the "t lines become "GC_t before being printed back out.

You can find more options like these in "perldoc perlrun", or Perl Command-Line Options.

Cheers