I would be interested to know why you feel you have to use IO::File. Is it perhaps mandated by your employers? Here is the script amended to use IO::File;. Only three lines have changed and I have left the original lines in comments. You should have been able to work this out from the documentation for yourself.

use strict;
use warnings;

use IO::File;

my $inFile = q{spw623147.txt};

my $inFH = IO::File->new($inFile, O_RDONLY)
#open my $inFH, q{<}, $inFile
   or die qq{open: $inFile: $!\n};

while ( $_ = $inFH->getline() )
#while ( <$inFH> )
{
    if ( m{\A\.rem} )
    {
        print qq{  Comment Line: $_};
        next;
    }
    if ( m{\A\.declare\s+(\S+)} )
    {
        print qq{     Data Line: $_};
        print qq{   Variable is: $1\n};
        next;
    }
    print qq{Not recognised: $_};
}

$inFH->close()
#close $inFH
   or die qq{close: $inFile: $!\n};
[download]

Cheers,

JohnGG