= can be used as a shortcut for the which command in zsh, e.g.:

$ echo $(which tmux)
/usr/local/bin/tmux
$ echo =tmux
/usr/local/bin/tmux

However, beware that it won’t work when used directly in an interactive command:

$ which uname
/usr/bin/uname
$ =uname
Darwin

And it seems to skip alias lookups (which I find nice!):

$ echo $(which ls)
ls: aliased to ls -G
$ echo =ls
/bin/ls