my %file1;

while (<IN1>) {
    next unless (/.:./);  # maybe warnings aren't needed here
    chomp;
    my ($key,$val) = split( /:/, $_, 2 );
    if ( $key =~ /^\s*$/ ) {
        warn "$inf1:$.:Empty key field\n";
        next;
    }
    if ( exists( $file1{$key} )) {
        warn "$inf1:$.:Duplicate key string\n";
        next;   # might want to say more about that
    }
    # might want to check $val too...

    # get here if everything's okay:
    $file1{$key} = $val;
}
close IN1;