perl -e '$|=1; print "hi\n"; $sec = 180; while ($sec--) { printf "Starting in %3s seconds\r", $sec; sleep 1 }'

Explanation:

• -e tells perl to execute the string following it as perl code
• $| = 1; tells perl to avoid buffering STDOUT, that is to make every print arrive to the screen immediately
• print "hi\n"; there only to be nice
• $sec = 180; sets the timer in seconds.
• The key however is that we print \r carrige return at the each of each print instead of \n newline. Thus each print overwrites the output of the previous one creating the effect of only the counter number changing.