I have been told it was possible, but without a single working example, that sed can read from a string variable without need for an input file. I have yet to get it to work.
For general safety, I’m writing the $PATH variable to another variable, as I mess with this, because I don’t need other problems to arise until I know exactly how to do this.

Consider the following:

~$x=$PATH

~$sed -i 's/:/ /g' $x

this fails with: No such file or directory.

Here are some others I have tried:

~$ sed -i 's/:/ /g' | (read x) 

sed: no input files

~$ sed -i 's/:/ /g' | (read $x) 

bash: read: `/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games': not a valid identifier

sed: no input files

~$ sed -i 's/:/ /g' | $(read x) 

sed: no input files

~$ sed -i 's/:/ /g' | $(read $x) 

bash: read: `/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games': not a valid identifier

sed: no input files

~$ sed -i 's/:/ /g' < $(read $x) 

bash: read: `/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games': not a valid identifier

bash: $(read $x): ambiguous redirect

~$ sed -i 's/:/ /g' < read $x 

bash: read: No such file or directory

~$ sed -i 's/:/ /g' < (read x) 

bash: syntax error near unexpected token `('

~$ sed -i 's/:/ /g' < $((read x)) 

bash: /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games: syntax error: operand expected (error token is "/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games")

~$ sed -i 's/:/ /g' < $((read $x)) 

bash: read /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games: division by 0 (error token is "usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games")

~$ sed -i 's/:/ /g' << $((read $x)) 

> ;^C

~$ sed -i 's/:/ /g' << $(read $x)

> ;^C

~$ sed -i 's/:/ /g' $x

sed: can't read /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games: No such file or directory

~$ sed -i 's/:/ /g' < $x

bash: /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games: No such file or directory

~$ sed -i 's/:/ /g' < echo $x

bash: echo: No such file or directory

~$ sed -i 's/:/ /g' |  echo $x

/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games

sed: no input files

Can this even work? I would prefer not to need to write files I don't need just so I can use sed. For this particular example, if ~$x=$PATH ~$sed -i 's/:/ /g' $x actually worked the way I would have hoped, I would get:

/usr/local/sbin /usr/local/bin /usr/sbin /usr/bin /sbin /bin /usr/games /usr/local/games

which I could then assign to a variable, and use in future commands, like ls $x