$ cat file.txt 
old/path/to/some/file
another/old/path/to/some/file
one/more/old/path/to/some/file

$ perl -pe 's#\bold(?=/path/to/some/)#new#g' file.txt 
new/path/to/some/file
another/new/path/to/some/file
one/more/new/path/to/some/file