You might use the function lc to lowercase the whole string (i.e. $input_command) before you split it.

You can read in a file line by line with  $line=<ADDYFILE> or in one single swoop with @file=<ADDYFILE>.

To go through @file you could use foreach:

foreach my $line ( @file ) {
  if ($line=m/^\s*$subname\s*:\s*(\S*)/i) { 
     do something, we found it 
  }
}
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I'm assuming here that the format of @file looks something like this:

submane1: address1
 subname2   :   address2
...
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and addresses have no spaces in them.