I was hoping that because I'm using in-line modification from the command line, that it could somehow be a 1-liner

REM Windows CMD prompt
perl -F: -e "$F[0] =~ s/-/_/g; print join(':',@F);"
[download]

# Linux/Unix/Cygwin shell
perl -F: -e '$F[0] =~ s/-/_/g; print join(":",@F);'
[download]

Disclaimer: Not tested.

Explanation: -F: turns on auto-split (-a) and auto-loop (-n) and sets the split character to ':'