Bingo! That was it. Thanks, oh venerable monks! Removed it by adding:

$hostname =~ s/\r//g;
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Now output shows:

Launching [[ssh -q user@host 'sed -i "s#.*HOSTA#ssh-rsa AAAAB3NzaC1yc 
+root@HOSTA#gI" /tmp/test.txt']] at ./key.pl line 81.
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And the command is doing what it should. To learn is to struggle.