use strict;
use warnings;
use IO::File;

{
    my $fh = IO::File->new( '< filename.txt' );
    if( defined $fh ) {
        while( my $line = <$fh> ) {
            print $fh->input_line_number(), 
                  ":\t",
                  $line;
        }
    }
}   # As $fh falls out of scope it is closed automatically.