my @MessageLines = ( 'Message Line 1', 'Message Line 2', 
    'Message Line 3',
    'Prompt:             ' );

if ( $MessageLines[-1] =~ m{ \A ( .* \S ) \s* \z }xms ) {
    my $CursorCol = length $1;
    print "\$CursorCol\[$CursorCol\]\n";
}