'unexpected token' in PowerShell when fully pathing executable

Question

Just trying to better understand why the second item below does not work. The first item is simple, the second seems clearer, the third seems unintuitive.

# My path includes pscp so this works.
pscp.exe -i $PRIVATE_KEY $file ${PROXY_USER}@${PROXY_HOST}:${PROXY_DIR}

# This does not work. I get unexpected token error. Why? What does that mean?
$PUTTY_PATH\pscp.exe -i $PRIVATE_KEY $file ${PROXY_USER}@${PROXY_HOST}:${PROXY_DIR}

# & is required to solve the problem.
& "$PUTTY_PATH\pscp.exe" -i $PRIVATE_KEY $file ${PROXY_USER}@${PROXY_HOST}:${PROXY_DIR}

Answer 1

That's because this is also considered a parse error:

"foo"\pscp.exe 

Whereas this parses correctly as you have found:

"$PUTTY_PATH\pscp.exe"

That resolves to a valid string but as you have already noticed, a string doesn't execute. You have to use the call operator & to invoke the command that is named by the string that follows.