First, to output to the browser as soon as the information come from script, set the autoflush to on. This can be done with:

$| = 1;
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To discover if the script is running from command line, an possible idea is testing the existance if any CGI environment variable, such as GATEWAY_INTERFACE.

if( defined $ENV{'GATEWAY_INTERFACE'} ) {
  # we are under CGI
} 
else {
  # we are under command line
}
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Hope this helps