If you wish to preserve the order found in the source file, put the hashes inside an array. If you don't care about order, or want them sorted, I'd put the arrays inside the hashes.

            my %inpinf = ();
            while (my $inpbuf = <INPFIL>)
            {
                chomp $inpbuf;
                my ($inpkey, $inpval) = split /\s*\:\s*/, $inpbuf, 2;
                push @{$inpinf{$inpkey}}, $inpval;
            }


C:\Steve\Dev\PerlMonks\P-2013-10-18@1437-DataMerge>type test2.dat
Some Field   : some value
Another Field: 1234
Different One: 5678
Yet Another  : foo
Another Field: 9012
Different One: 3456
Yet Another  : bar

C:\Steve\Dev\PerlMonks\P-2013-10-18@1437-DataMerge>datamerge.pl test2.
+dat
Processing "test2.dat"
%inpinf (C:\Steve\Perl/debug.pm:887(990)):
    [Another Field] => [ARRAY(0x4a8498)]
        [1234]
        [9012]
    [Different One] => [ARRAY(0x4bd6a8)]
        [5678]
        [3456]
    [Some Field] => [ARRAY(0x4a83f0)]
        [some value]
    [Yet Another] => [ARRAY(0x4bd768)]
        [foo]
        [bar]
[download]

Full code (apologies for using my home-spun debugmodule rather than Data::Dumperbut I haven't switched over to  Data::Dumperyet and wanted a fast way to show you the resulting data structure.)

#!/usr/bin/perl

use strict;
use warnings;

use debug;


foreach my $inpfnm (@ARGV)
{
    print "Processing \"$inpfnm\"\n";

    if (!open INPFIL, '<', $inpfnm)
    {
        print "ERROR:  Cannot open input file \"$inpfnm\"\n";
    }
    else
    {
        my $outfnm = $inpfnm . '.out';
        if (!open OUTFIL, '>', $outfnm)
        {
            print "ERROR:  Cannot open output file \"$outfnm\"\n";
        }
        else
        {
            my %inpinf = ();

            while (my $inpbuf = <INPFIL>)
            {
                chomp $inpbuf;
                my ($inpkey, $inpval) = split /\s*\:\s*/, $inpbuf, 2;
                push @{$inpinf{$inpkey}}, $inpval;
            }
            close OUTFIL;

            &debug::predebugdumplist("\%inpinf", \%inpinf);
        }
        close INPFIL;
    }
}

exit;

__END__
[download]