seq_1:
    1    33     gene
    1    20     exon
   21    27     exon
   28    33     exon

seq_2:
    1    80     gene
    1    80     exon

seq_3:
    1    55     gene
    1    30     exon
   31    50     exon


##</code><code>##

#!/usr/bin/perl -w
use strict;

my %seq_info;
my $in_file = 'datafile.txt';
#open $INFILE, '<', $in_file or die "Could not open '$in_file':  $!\n";

#while ( <$INFILE> ) 
while ( <DATA> ) 
{
    my @record = split( /\s+/, $_ );
    push @{ $seq_info{ $record[0] }  }, [ @record[1..3] ];
}

foreach ( sort keys %seq_info ) {
    print_seq_chunk( $_, $seq_info{$_} );
    print "\n";
}

sub print_seq_chunk {
    my ( 
        $seq_id,
        $seq_info_ar
            ) = @_;

    print $seq_id, ":\n";
    printf "  %3d   %3d   %6.6s\n", @$_
        foreach @$seq_info_ar;
}


__DATA__
seq_1    1    33    gene
seq_1    1    20    exon
seq_1    21    27    exon
seq_1    28    33    exon
seq_2    1    80    gene
seq_2    1    80    exon
seq_3    1    55    gene
seq_3    1    30    exon
seq_3    31    50    exon